Writing a single unit test for multiple implementations of an interface

Question

I have an interface List whose implementations include Singly Linked List, Doubly, Circular etc. The unit tests I wrote for Singly should do good for most of Do

Answer 1

I had exactly the same problem and here is my approach with help of JUnit parameterized tests (based on @dasblinkenlight's answer).

1. Create a base class for the all test classes:

@RunWith(value = Parameterized.class)
public class ListTestUtil {
    private Class listClass = null;

    public ListTestUtil(Class listClass) {
        this.listClass = listClass;
    }

    /**
     * @return a {@link Collection} with the types of the {@link List} implementations.
     */
    @Parameters
    public static Collection> getTypesData() {
        return List.of(MySinglyLinkedList.class, MyArrayList.class);
    }

    public  List initList(Object... elements) {
        return initList(Integer.class, elements);
    }

    @SuppressWarnings("unchecked")
    public  List initList(Class type, Object... elements) {
        List myList = null;
        try {
            myList = (List) listClass.getDeclaredConstructor().newInstance();
            for (Object el : elements)
                myList.add(type.cast(el));
        } catch (Exception e) {
            e.printStackTrace();
        }
        return myList;
    }
}

2. Classes the contain test cases extend ListTestUtil and you can just use initList(...) wherever you want:

public class AddTest extends ListTestUtil {
    public AddTest(Class cl) {
        super(cl);
    }

    @Test
    public void test1() {
        List myList = initList(1, 2, 3);
        // List myList = initList(Strng.class, "a", "b", "c");
        ...
        System.out.println(myList.getClass());
    }
}

The output proves that the test is called twice - once for each implementation of the list:

class java.data_structures.list.MySinglyLinkedList
class java.data_structures.list.MyArrayList